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Using ultrasound measurements when selecting replacement rams can make our lamb industry more competitive.

By John E. Hall

The project was funded by a SARE farmer grower grant

The ram you select today will influence your flock this year and for years to come if you retain any females. Carcass traits should be your primary concern, regardless of breed if the American lamb is going to retain its market value, shepherds must market lambs equal to imported lambs.

I recently had the opportunity to eat New Zealand lamb at a nearby business dinner party. It was excellent. I was so impressed; I asked the host if he had any remaining lamb chops in the freezer. He did. I went by and traced the loin surface area of the chop. They measured 3.1 square inches. I participate in the Pennsylvania ram test annually. The rams on test are ultrasounded and the rib eye muscle is measured prior to the sale.

In 2004, the Suffolk junior ram lambs at the ram test scanned 2.49 square inches adjusted to 125 pounds. When I removed the top ram, the average dropped to 1.94 square inches. I have been measuring the rib eyes of the lambs in my flock for the past 10 years. Progress has been frustrating. The New Zealand lamb experience triggered my interest in researching and publishing data about a non-evasive tool that can be used to measure muscle, i.e. rib eye area (REA) in rams. Research has shown that REA is highly heritable. This means that the wrong ram can reduce REA and fast as a superior ram can increase REA.

Ultrasound technology has been used with beef and swine for years. Great strides have been made using ultrasound to measure REA in these species. How does it work? In the normal scenario for estimating carcass traits via ultrasound, a "Certified Technician" travels to a designated location with portable ultrasound equipment.

Upon restraint of an animal, the technician would apply a "couplant" (usually vegetable oil) to the back of the animal at a designated location. The couplant prevents the interference of air between the transducer and the animal. This allows for maximum conduction of sound waves. Real-time ultrasound allows an image to be produced immediately. This image can be captured onto a computer's hard drive, allowing for the images to be interpreted at a later time.

Ultrasound measurements for backfat thickness (BF) and longissimus dorsi area (Ribeye area, REA) are taken between the 12th and 13th ribs on each animal. A different ultrasound measurement for intramuscular fat (Marbling/Percent Intramuscular Fat, PIMF) is also taken across the area between the 12th and 13th ribs parallel to the spine. Once the image is captured, the technician traces the muscle and a computer program calculates the cross section area of the muscle.

This research project was intended to show that selecting rams with larger than average REA, one could increase the REA / carcass traits in the flock.

Iowa State University has developed a regression equation to use with ultrasound data. It adjusts the REA to 125 pounds (or market weight). I have used this equation to adjust the scanned REA to a given weight in my flock for the past 10 years. I had thought that I could use this adjustment equation at various weights. In fact, I wanted to scan the sheep at a light weight ( 45-60 pounds) select the replacements, then offer the rest of the lambs for sale to 4-Hers and others looking for lambs.

Since I have a small flock of 20 to 25 ewes, I chose to scan at three different weights to add to the data points for my research. What I found out was that the ultrasound measurement had to be completed at or near the finish weight to be relevant. I scanned the lambs on May 9 when they averaged 65 pounds, again in June when they averaged 95 pounds, and the third time in July when they averaged 130 pounds. The data is shown in the table below

Table 1 - Summary of scanning lambs at three different weights

Date	No. scanned	Avg. weight	Actual REA	backfat	adj. REA
May 9	20	65.4	1.83	0.10	2.61
June 20	16	94.9	2.54	0.12	3.17
Sept. 12	12	129.7	3.24	0.22	3.33

Although this project did not prove that selecting rams for muscle thickness would increase REA of the flock, it did show that one must ultra sound very near the final weight to get the true information.